The present invention relates generally to measurement devices and more particularly to an automatic wire (cable) dielectric analyzer that determines the dielectric constant for a wire having a known length. The determined dielectric constant can be used by a Time Domain Reflectometer (TDR). As used herein, the terms wire and cable are interchangeable.
When two metallic conductors are placed close together they form a transmission line that has characteristic impedance. A Time Domain Reflectometer (TDR) looks for a change in impedance that can be caused by a variety of circumstances, for example, cable damage, water ingress, change in cable type, improper installation or manufacturing flaws.
A TDR works on the same principle as radar ranging. A pulse of energy is transmitted down a cable under test. When the pulse reaches the end of the cable or a fault along the cable, part or all of the pulse energy is reflected back to the TDR. The TDR measures the time that it takes for the signal to travel down the cable and reflect back. This time is then converted to distance. The distance can then be displayed either as a textual distance reading or as a waveform.
The TDR is a very accurate instrument, however, variables in the cable itself can cause errors in the distance measurements. This error can be minimized by using a correct Velocity of Propagation (VOP) of the cable under test. The VOP is the speed at which a signal travels through the cable. The speed of light in a vacuum is 186,400 miles per second. If the speed of light is represented by the number 1 or 100%, the VOP is expressed as a percentage of the speed of light. For example, if a signal travels through a cable at 65% of the speed of light, that cable has a VOP of 0.65. Different types of cables have different VOPs.
It is important to have the correct VOP when using a TDR for fault finding. Typically, the VOP is listed in the cable manufacturer's catalog or specification sheet. However, this information is not always readily available when testing a cable. Furthermore, variations in the VOP of the same type of cable are not uncommon. For example, the VOP of a cable may change with temperature, age and humidity. The VOP may also be different for different manufacturing runs.
In order to operate a TDR and determine a distance to a fault, the dielectric constant of a wire under test must conventionally be known. As described above, it is also important to have the VOP. Like the VOP, the dielectric constant of the wire under test is usually acquired from the wire manufacturer. However, like the VOP, the dielectric constant or assurances of its reliability are often unavailable when a cable is being tested and a trial and error manual procedure is typically used to determine the dielectric constant when using a TDR. This manual trial and error process involves initially entering into the TDR a “best guess” for the dielectric constant for the cable under test. A cable under test of known length is then connected to the TDR. Markers are manually placed at the open circuit point of the TDR test cable and the open circuit point of the cable under test. The distance of the cable under test is then manually read from between these markers from the acquired waveform. Typically, there is an error in this distance compared to the known length of the cable under test. The operator then repeatedly adjusts the dielectric constant and takes subsequent TDR acquisitions and readings until the TDR distance reading of the waveform of the cable under test equals the actual known distance of the cable under test. At this point the dielectric constant is known. This is an iterative, time-consuming manual procedure that also requires analysis of TDR waveforms. A typical manual trial and error operation as described above could take about fifteen to twenty minutes to acquire an accurate reading of a dielectric constant for a cable under test.
Due to the problems described above, there is a need to automatically and accurately determine the VOP and/or dielectric constant for a cable having a known length.